We propose basic photochemical studies in three areas, bilirubin and other heterocyclic compounds, steroids and related systems, and asymmetric induction. Our long range goal is a molecular level understanding of the chemical, photochemical, and photophysical processes involved in health problems in which ultraviolet or visible light plays a significant role. The studies proposed should make direct contributions to understanding the phototherapy of neonatal jaundice (photodestruction of bilirubin by irradiation of the infant) and the photochemical origins of skin photosensitization by medicinal agents. Emphasis will be placed on the aerobic and anaerobic photochemistry of bilirubin, definition of primary photochemistry, relationship between observed photochemistry and nature of the excited state, wavelength dependence of stereochemistry in the opening of dienes (related to vitamin D chemistry), the photochemical transformations of delta 4-3-ketosteroids, and absolute asymmetric induction using circularly polarized light. Our low temperature (1.3-80 degrees K) techniques provide novel approaches to determining the primary photochemistry of molecules and to the synthesis of reactive molecules such as oxarene, thiarene, and azarene.